1. Field of Invention
This invention relates to melt-processable, radiation crosslinkable, fluorocarbon polymer compositions.
2. Description of the Prior Art
It has been proposed to irradiate fluorocarbon polymers to improve their mechanical properties at elevated temperatures. Suitable ionizing radiation includes gamma rays and accelerated electrons. As a rule, degradation, rather than crosslinking, predominates when fluorocarbon polymers are subjected to ionizing radiation. It has also been proposed to heat anneal such polymers subsequent to irradiation to improve their mechanical properties. It also has been reported that a small amount of an unsaturated compound such as triallyl cyanurate (TAC) functions in fluorocarbon polymers as a crosslinking promoter when such formulations are exposed to radiation. The prior art teaches that incorporation of these agents in polymers above a critical concentration, usually about 4% by either melt mixing or by imbibition does not lead to a further increase in crosslink density upon subsequent irradiation.
It has also been reported that fluorocarbon polymer compositions containing relatively volatile crosslinking promoters such as triallyl cyanurate or its isomer, triallyl isocyanurate, cannot be processed, such as by extrusion or injection molding, when the fluorocarbon polymer composition requires a processing temperature above about 250.degree. C. For a variety of fluorocarbon polymers, temperatures above 250.degree. are required to fabricate shaped articles such as wire insulation, sheets, film, tubing, gaskets, and boots. Melt processed polymer compositions containing prior art crosslinking agents tend to prematurely crosslink and to form gels or lumps, discolor and often to form voids in the final product. As set forth in detail in U.S. Pat. Nos. 3,763,222, 3,840,619; 3,894,118; 3,911,192; 3,970,770; 3,985,716; 3,995,091, and 4,031,167, substantial difficulty has been experienced in providing heat stable crosslinking agents of low volatility suitable for use with fluorocarbon polymers. There is no known crosslinking agent which provides entirely satisfactory properties in melt processed irradiated fluorocarbon polymer compositions.